This invention relates to the purification of acrylonitrile. More particularly, it relates to removal of acrolein from acrolein-containing acrylonitrile process streams by reaction of the acrolein with a scavenger compound to produce an acrolein derivative.
Acrylonitrile is a well-known article of commerce widely used in the manufacture of synthetic resins and fibers and as a valuable intermediate in the synthesis of many organic compounds. In most large-scale industrial processes for producing this nitrile, minor amounts of undesired byproducts and contaminants, including acetone, acrolein, acetaldehyde, and other similar carbonyl compounds, are simultaneously produced.
In most commercial applications utilizing acrylonitrile as a starting material, it is critical that the starting material be in as pure a state as possible as even minute traces of impurities can cause extremely low production yields and/or inferior end product quality. This impact is particularly prevalent when acrylonitrile is used in the preparation of synthetic resins and fibers.
The presence of even very small trace quantities of acrolein renders the acrylonitrile unsuitable for many applications. For example, acrolein can act as a crosslinker or an alkylol adduct with acrylamide when the nitrile is converted to acrylamide and then polymerized, such as for eventual end-use as a fiber or flocculent. The presence of acrolein contaminant can thereby result in polyacrylamide that is of insufficient molecular weight or which contains undesirable insoluble material. Thus, it is essential for many end uses that acrolein concentrations in acrylonitrile starting materials, if any, be reduced to below 15 ppm, more preferably 5 ppm or less, by weight of the commercial grade acrylonitrile if it is to be successfully marketed.
Although prior art commercial acrylonitrile production systems typically include purification steps often using distillation processes, acrolein reduction to this required level is difficult. More specifically, prior art methods for removal of acrolein from the crude acrylonitrile stream have typically involved pH adjustments and/or control at various points in the process which add cost or have undesirable impact such as side reaction catalysts and equipment fouling. In U.S. Pat. No. 3,185,636 to Stevens et al, the absorber column pH is maintained at a substantially neutral or slightly alkaline pH, whereby the saturated carbonyl compounds in the reactor effluent combine with the excess hydrogen cyanide to form the corresponding cyanohydrin. In U.S. Pat. No. 3,462,477 to Caporali et al a pH of between 7.5 and 11 was required to separate the acrolein from the crude acrylonitrile by distillation. In published EP application 0110861, acrolein is removed by maintaining the pH in the zone of maximum acrolein concentration of the recovery column at from about 5.25 to 7.
Other attempts to separate acrolein and other byproducts from acrylonitrile have not successfully reduced the acrolein content to the required level. In U.S. Pat. No. 3,328,266 to Modiano et al the crude acrylonitrile stream from the absorber with a pH generally between 9 and 9.5 is subjected to extractive distillation apparently without any further adjustment of pH. The acrolein content remains high, however, even after the third stage of the distillation is completed and does not produce acrylonitrile of sufficient purity for fiber production.
The separation of acrylonitrile from acetonitrile and small quantities of acrolein is discussed in U.S. Pat. No. 3,459,639 to Borrel et al. Under the disclosed conditions for the distillation recovery column, the acrolein was not significantly affected and remained above 200 parts per million in the separated organic product phase.
Yet other prior art acrolein removal processes utilize additives to improve and increase acrolein reduction. For example, U.S. Pat. No. 5,760,283 describes the addition of strong bases in the recovery section of the process while U.S. Pat. No. 6,074,532 describes adding a substituted aromatic amine prior to distillation. These types of processes add significant cost to the overall process and have the potential to catalyze unwanted side reactions and to form undesirable byproducts.
Japanese application no. 53-60040 describes the removal of acrolein with an ion exchange resin. Accordingly, there continues to be an unmet need for an inexpensive and effective process for removal of acrolein from process streams that achieves the purity levels critical for use of these process streams.